In line with a global trend towards improved energy efficiency and energy conservation, the lighting industry is moving from incandescent lighting to alternative, more energy-efficient light sources. Solid-state light-emitting devices (LEDs), such as light-emitting diodes, have evolved rapidly in recent years and are now available with high brightness and suitable color rendering indices (CRI) for replacement of incandescent lamps, while providing improved energy efficiency and operational lifetime. LEDs are available for solid-state lighting (SSL) applications for both domestic and commercial use. The compact size of LEDs allows for lamps of different form factors from traditional incandescent lamps and fluorescent light fixtures. However, there is ongoing demand for replacements for conventional incandescent bulbs of standard configurations, i.e. for existing light fixtures.
For example, one of the most commonly used bulbs is the A19 bulb configuration that provides a generally omni-directional light distribution. This is the form of a conventional pear-shaped incandescent light bulb with a tungsten filament. Other common configurations are candelabra bulbs, which produce a narrower distribution, and parabolic reflector bulbs, which produce a more collimated beam for spotlights or down lighting.
LED light sources may comprise a die with a single light-emitting element, or an array of multiple light-emitting elements, with or without a hemispherical dome lens. Currently, white light LED light sources for solid-state lighting typically comprise a substrate carrying a planar array of a plurality of LED emitters covered by a layer of a phosphor material to provide light emission of a desired CRI and spectral distribution over the visible wavelength range. The light-emitting area may, for example, be a flat circular area, about 10 mm in diameter, comprising many light emitters, carried on a rectangular chip or substrate. Light emitted from the flat surface of the LED light source is therefore emitted over an angular distribution of up to 180°, i.e. in a hemispherical distribution (2π steradian). This is significantly different from that of the filament of an incandescent light bulb, which emits over a spherical, 360° angular distribution (4π steradian).
For solid-state light sources designed to replace conventional incandescent light bulbs, it is desirable to increase the angular distribution of light emitted from the surface of an LED light source to provide an omni-directional distribution closer to that of an incandescent bulb. Thus, typically, one or more optical elements, such as lenses, collimators, light guides or light pipes are used to extract and distribute light emission from the surface of an LED light source.
Where it is desired to provide an LED light source to replace a conventional incandescent light bulb, such as a standard bulb or candelabra bulb, it is known to use a light pipe or light guide to transport light some distance from the LED surface, and then distribute the light into a more omni-directional distribution, e.g. by a process of reflection and/or refraction and/or total internal reflection (TIR) to emulate the 360° angular distribution of a filament of a conventional incandescent light bulb. Such a light guide may be referred to as a “virtual filament” or “solid-state filament”.
Numerous designs have been proposed for light pipes or light-guiding devices for transporting light from the LED light source with low loss of luminous intensity.
Light may be collimated or concentrated into a light guide, e.g. using a conical or tulip shaped collimator (see for example, U.S. Pat. No. 6,547,423 or US Patent Publication No. US2011/0051394). Light is transmitted or propagated along a length of a light pipe or a light guide by means of total internal reflection (TIR). Light exits from a surface or surfaces of the light pipe by reflection, refraction or scattering. The geometry of the exit surfaces of the light guide determines the angular distribution of light emitted. The light guide may be made from transparent optical-grade glass or polymer material, for example. Losses can occur from reflection and refraction at multiple interfaces, and losses may also be dependent on the thickness and optical properties of the material of the light guide, e.g. absorption losses. The following references provide a few examples of light guides of this type:
U.S. Pat. No. 7,753,561 and U.S. Pat. No. 7,329,029 to Chaves et al., both entitled “Optical Device for LED Lamp” and related patents cited therein, disclose elongated light guides, which comprise a transfer section and an ejector section. The transfer section is referred to as a compound elliptical concentrator, and the ejector section comprises reflection and refraction surfaces to produce a more spherical distribution of light, e.g. for conventional light bulb configurations. These references also disclose use of a TIR lens in combination with a light guide to further shape the light distribution.
U.S. Pat. No. 7,006,306 to Falicoff et al., entitled “Circumferentially emitting luminaires and lens-elements formed by transverse-axis profile-sweeps” discloses lens elements or light guides of various configurations, including some with a generally conical or cylindrical form, having inner and outer surfaces with multiple multi-directional facets to form a flattened, narrow output beam around 360° (similar to the sweep of a lighthouse beam), e.g. for illuminating signage.
U.S. Pat. No. 8,215,802 to Bailey, entitled “Multiple Tier Omni-directional Solid-State Emission Source” discloses a light guide reported to have a high optical transmission. The optic provides for collimation of light from an LED light source at the bottom of the optic, delivery of light to a position at a distance from the source, and at the top, a complex tiered structure with reflection and refraction surfaces to provide an omni-directional light distribution.
Although some of the light guides disclosed in these references provide for omni-directional light distributions over a very wide angle, the geometry of these structures is complex. The multiple facets with multi-directional forward and backward facing surfaces make these structures complex and expensive to design and manufacture, requiring complex mold tools, multistep molding and finishing processes to produce the product. Complex shapes with multiple opposing surfaces may require multi-part molds with seams, for example, that necessitate finishing or polishing of optical surfaces of the light guide after the molding process, which adds to manufacturing costs.
Apart from design and manufacturing issues, these complexly-shaped light guides have multiple reflective/refractive surfaces or facets that intersect at acute angles. The sharp angles, between opposing reflective surfaces can generate significant discontinuities or undulations in the angular distribution of the light intensity. These discontinuities may be apparent as artifacts in the light distribution, such as brighter and darker fringe patterns that are visible to the human eye and/or which may be unacceptable for applications requiring uniform or even lighting. Thus, although many light guides, light pipes, and non-imaging optics have been proposed to provide a wider angular distribution from an LED light source, many do not meet the requirements established by existing lighting standards and/or are complex and expensive to manufacture.
Another issue is that in the above referenced structures, the light guide typically comprises a transparent optical grade polymer material, such as PMMA (polymethyl methacrylate) that can degrade during extended use at high temperatures. The light guide is positioned over the light-emitting surface, close to, or in contact with, the LED chip. Operating temperatures of the LED emitter surface may typically rise to 100° C. or more. Therefore, the polymer material is exposed to elevated temperatures that contribute to aging or premature breakdown of the material, which over time can degrade the optical properties, such as the refractive index, and decrease the useful operational life span. Existing light guides do not effectively address these issues of thermal stresses or thermal degradation.
Thus, there is a need for improved or alternative optical elements or light guides that address one or more issues of cost, manufacturability, low transmission efficiency, limited luminous efficiency and uniformity, and poor thermal management.
In particular, there is a need for improved or alternative optical systems and devices for solid-state lighting systems comprising LEDs, which can replace conventional incandescent light bulbs, e.g. for use in SSL applications requiring light sources which have an omni-directional light distribution.